1. Field of the Invention
This invention relates to flame retardant polyorganosiloxane foams. More particularly, this invention relates to polyorganosiloxane foams that exhibit a high resistance to burning and disintegration when exposed to the extreme heat generated by flames, e.g. 2000.degree. C. and higher, and a preferred method for preparing these foams.
2. Description of the Prior Art
Polyorganosiloxane foams can be rendered flame retardant using materials such as platinum and carbon black that do not evolve noxious or toxic vapors at elevated temperatures. Some polyorganosiloxane foams of this type will form a compressive seal when the ingredients employed to prepare the foam are reacted in a cavity or passageway, which makes these foams desirable for use as penetration seals in buildings and other types of construction.
In many buildings, particularly of the multifamily residential or commercial type, the walls and floors contain numerous passages connecting adjacent sections of the building. These passages are used for pipes, electrical conductors, ventilation ducts and conduits. The remaining space in these passages is usually occupied by solid materials referred to as penetration seals. In the event of a fire in one section of the building, the penetration seals are intended to prevent the passage of smoke, other vapors, and water into adjoining sections. In addition, the seals should be sufficiently flame retardant that they will not permit the flames in one section to spread to adjoining sections until the adjoining sections can be safety evacuated. To function effectively as a penetration seal and fire stop, the polyorganosiloxane foam should withstand prolonged contact by flames having temperatures such as 2000.degree. C. before the flame can completely burn through the seal.
Various methods for preparing flame retardant polyorganosiloxane foams are described in the prior art. Ronald L. Joyce, in U.S. Pat. No. 2,956,032, issued on October 11, 1960, discloses elastomeric polyorganosilane foams containing nickelous bromide, pentabromoethylbenzene, or pentabromotoluene as flame retarding agents. The use of mixtures containing asbestos and fibrous potassium titanate as flame retarding additives for polyorganosiloxane foams is taught in U.S. Pat. No. 3,425,967, issued on February 4, 1969, to Modic. Lee and Ronk in U.S. Pat. No. 4,026,842, issued on May 31, 1977, and Smith in U.S. Pat. No. 3,923,705, issued on Dec. 2, 1975, employ platinum or a platinum compound to improve flame retardancy of polyorganosiloxane foams prepared by reacting organohydrogen siloxanes and siloxanes containing silicon-bonded hydroxyl groups. Smith teaches that flame retardancy can be further improved by the optional addition of carbon black.
U.S. Pat. No. 4,082,702, issued on April 4, 1978, to Harper, discloses syntactic polyurethane foams containing conventional flame retarding agents, such as tris(2-chloroethyl)phosphate. The flexural strength of the foam is increased by the presence of noncombustible flexible fibers, such as glass fibers, in addition to microballoons, also referred to as hollow microspheres, of glass or other material employed to impart a cellular structure to the foam. Example 1 of this patent demonstrates the requirement for a conventional flame retarding agent. When the flame retarding additive was omitted, the limiting oxygen index, which is directly proportional to the flame retarding ability of the material, decreases from 80% oxygen to 17% oxygen.
As used in the context of many of the aforementioned patents, the term "flame retarding" refers to materials which are self-extinguishing, in that they continue to burn for at most a limited amount of time, if at all, once the flame used to ignite the foam sample is removed. A frequently employed test method to evaluate this type of flame retardancy for polymeric materials is "The Standard For Tests For Flammability of Plastic Materials for Parts in Devices and Appliances, UL94," published on June 1, 1973, by Underwriters' Laboratories, Inc. In accordance with this procedure, a sample is contacted with a gas-fired flame from a bunsen burner for 60 seconds. The flame is then removed from the area of the sample and the time required for the burning sample to extinguish itself is measured. The aforementioned patent to Smith reports that the foam prepared using the formulation described in Example 1 of this patent burned for less than 2 seconds after the flame was removed and the length of the sample that burned was less than 1.27 cm. This test provides no indication of how long it will take for a flame applied against one surface of a relatively thick foam sample to reach the opposite side of the foam.
A test method that provides an indication as to whether a material is sufficiently flame retardant to form an effective penetration seal is described in ASTM test method E-119, published by the American Society for Testing of Materials. In accordance with this test, a surface of the sample of material to be evaluated is contacted with an open flame and the time required for the flame to burn through a specified thickness of the sample is measured.
Hitchcock, in U.S. Pat. No. 4,259,455, issued on Mar. 3, 1981, teaches the use of fibrous aluminum silicate to prepare polysiloxane foams which pass the ASTM E-119 test by being able to withstand flame temperatures of up to 2000.degree. F. (1090.degree. C.) for three hours without burning through. In addition to a fibrous aluminum silicate, the reaction mixtures also contain an organohydrogensiloxane, a hydroxyl-containing organosiloxane and a platinum catalyst. Hitchcock states that foams prepared using fibrous aluminum silicates require a thickness of from five to six inches (13-15 cm.) to pass the aforementioned ASTM El-119 test whereas prior art foams, including those disclosed in the aforementioned U.S. Pat. No. 3,923,705, to Smith, must be from 9 to 12 inches (24-30 cm.) thick to pass this test.
It has now been found that polysiloxane foams exhibiting a greater resistance to burning through than has heretofore been possible using prior art methods and materials can be prepared by incorporating into the cured foam effective amounts of finely divided fibrous and cellular forms of heat resistant, nonmetallic materials such as glass that do not soften or decompose below about 2000.degree. C., and platinum in elementary or chemically combined form.